An electrodynamic transducer may be utilized as a loudspeaker or as a component in a loudspeaker system to transform electrical signals into acoustical signals. An electrodynamic transducer typically includes a frame, a magnetic motor assembly that provides a magnetic field across an air gap, a voice coil, a diaphragm, and a suspension system coupled between the outer perimeter of the diaphragm and the outer perimeter of the frame. The voice coil, supported by a former, is coupled to the diaphragm so that the electrical current that flows through the voice coil causes the voice coil to move in the air gap and also causes the diaphragm to move.
The motor assembly typically includes a magnet and associated ferromagnetic components-such as pole pieces, plates, rings, and the like arranged with cylindrical or annular symmetry about a central axis. The voice coil typically is formed by an electrically conductive wire cylindrically wound for a number of turns around the lower portion of the voice coil former, while the upper part of the voice coil former is attached to the diaphragm. The coil former and the attached voice coil are inserted into the air gap of the magnetic assembly such that the voice coil is exposed to the magnetic field established by the magnetic motor assembly. The voice coil may be connected to an audio amplifier or other source of electrical signals that are to be converted into sound waves.
In a conventional construction, the diaphragm includes a flexible or compliant material that is responsive to a vibrational input. The diaphragm is suspended by one or more supporting but compliant suspension members such that the flexible portion of the diaphragm is permitted to move. In common constructions, the suspension members may include an outer suspension member known as a surround. The surround is connected to the diaphragm's outer edge and extends outward from the diaphragm to connect the diaphragm to the frame. The supporting elements may also include an inner suspension known as a spider. The spider is typically connected to the voice coil and extends from the voice coil to a lower portion of the frame, thus connecting the voice coil to the frame. In this way, the diaphragm is mechanically referenced to the voice coil, typically by being connected directly to the former on which the voice coil is supported.
In operation, electrical signals are transmitted as an alternating current through the voice coil, and the alternating current interacts with the magnetic field in the magnetic air gap. The alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air gap and, correspondingly, move the diaphragm to which the coil (or coil former) is attached. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves through a suitable fluid medium such as air.
Because the material of the voice coil has an electrical resistance, some of the electrical energy flowing through the voice coil is converted to heat energy instead of sound energy. The heat emitted from the voice coil may be transferred to other operative components of the loudspeaker, such as the magnetic assembly and coil former. The generation of resistive heat is disadvantageous for several reasons. First, the conversion of electrical energy to heat energy constitutes a loss in the efficiency of the transducer in performing its intended purpose—that of converting the electrical energy to mechanical energy utilized to produce acoustic signals. Second, excessive heat may damage the components of the loudspeaker and/or degrade the adhesives often employed to attach various components together, and may even cause the loudspeaker to cease functioning.
Thus, the generation of heat limits the power handling capacity and distortion-free sound volume of loudspeakers as well as their efficiency as electro-acoustical transducers. Such problems are exacerbated when one considers that electrical resistance through a voice coil increases with increasing temperature. That is, the hotter the wire of the voice coil becomes, the higher its electrical resistance becomes and the more heat it generates.
The most common form of a loudspeaker uses a single voice coil winding in a single magnetic gap. However, loudspeaker performance may be enhanced by using a multiple coil/multiple gap design. A multi-coil transducer may include two or more separate windings axially spaced apart from each other to form two or more coils, although the same wire may be employed to form the coils. The multiple voice coils are usually electrically connected together either on the coil itself or on the outside of the loudspeaker so that the coils work together to move the diaphragm. As both coils provide forces for driving the diaphragm, the power output of the loudspeaker may be increased without significantly increasing size and mass. The most common implementation of the multiple coil loudspeaker uses two voice coils and two magnetic gaps.
Many multi-coil/multi-gap designs are able to produce more power output per transducer mass and dissipate more heat than conventional single-coil designs. For example, a dual-coil design provides more coil surface area compared with many single-coil configurations, and, thus, ostensibly is capable of dissipating a greater amount of heat at a greater rate of heat transfer.
While the multiple coil/multiple gap construction has several advantages over single gap designs including higher power handling, reduced distortion, reduced inductance, and extended frequency response, there are at least three particular disadvantages with dual coil/dual gap speakers. First, insofar as a desired advantage of the dual-coil driver is its ability to operate at a greater power output, so operating the dual-coil transducer at the higher power output concomitantly causes the dual-coil transducer to generate more heat. Hence, the improved heat dissipation inherent in the dual-coil design may be offset by the greater generation of heat. There can be problems with overheated magnets due to the compact motor and the proximity of the magnets to the heat-generating voice coils. For example, as compared to single-coil transducers, adequate heat dissipation in many dual-coil transducers, and more generally multiple-coil transducers, continues to be a problem due to the longer thermal paths that must be traversed between the heat source (primarily the voice coil) and the ambient environment.
In attempt to provide transducer cooling, the pole piece may be formed with a center vent which provides a flow path for the transfer of cooling air from outside of the transducer. Air flow through this vent is created in response to movement of the diaphragm with the excursion of the voice coil. However, such designs do little to directly cool the transducer voice coil, as air is simply pumped straight through the pole piece out the back of the motor. In fact, in some cases, a very large center vent can reduce convective cooling in proximity of the voice coil, and therefore reducing power handling of the transducer.
In some instances, holes or slots may be formed radially within the pole piece and extend outwardly from the center vent toward the voice coil in an attempt to provide convective cooling to the voice coil. Such radial holes may be effective to cause cooling air from the center vent to flow directly against at least a portion of the voice coil, but the position and shape of these holes or slots does not efficiently pull toward the voice coil and disturbs the laminar air flow within the center vent, creating turbulence and drag. Furthermore, an acoustic problem can be created with such radial slots, as a large amount of air is forced through a small passage.
One method to attempt to more directly cool the voice coil is by forcing air through the narrow magnetic gap between the voice coil and motor at high velocity. This results in a forced air cooling of the voice coil, but then a forced air transfer of heat to the magnet parts. A method to cool the voice coil directly as well as the overall transducer is to transfer heat directly from the voice coil to the ambient air, skipping the magnet subassembly entirely. This can be accomplished by forcing air past the hot voice coil through the magnetic gap and exhausting it through vents to the ambient. However, a high velocity of air is desired in the gap, and such vents that connect directly to the magnetic gap can be quite noisy.